Field of the Invention
The present invention relates to a hydraulic drive system.
Background Information
Work machines, such as a hydraulic excavator or a wheel loader, are equipped with a hydraulic cylinder. Hydraulic fluid discharged from a hydraulic pump is supplied to the hydraulic cylinder through a hydraulic circuit. For example, Japanese National Publication of International Patent Application No. 2009-511831 describes a work machine equipped with a hydraulic closed circuit for supplying hydraulic fluid to the hydraulic cylinders. Kinetic energy and potential energy of the members driven by the hydraulic cylinder are regenerated due to the hydraulic circuit being a closed circuit. As a result, fuel consumption of a driving source for driving the hydraulic pump may be reduced.
A charge circuit is often installed in the closed hydraulic circuit. The charge circuit is provided for replenishing an amount of hydraulic fluid corresponding to oil leakage from the hydraulic pump. A charge pump and a relief valve are provided in the charge circuit. The charge pump is normally a fixed displacement pump and is driven by a driving source, such as an engine. The relief valve regulates the hydraulic pressure (referred to below as “charge pressure”) in the charge circuit. When the flow rate of the hydraulic fluid supplied to the hydraulic pump is insufficient, the hydraulic pressure in the hydraulic closed circuit falls below the charge pressure and hydraulic fluid is supplied from the charge circuit to the hydraulic closed circuit.